Semiconductor lasers are widely used in applications such as optical communications. The edge emitting laser diode is a semiconductor laser that emits light from a plane which is a continuation of the p-n junction of the diode. Cleaved surfaces at the ends of the diode act as mirrors which together define an optical cavity. Optical feedback provided by the cleaved mirrors creates a resonance of the emitted light that results in lasing.
The vertical cavity surface emitting laser (VCSEL) is another type of semiconductor laser in which the optical cavity is normal to the p-n junction of the semiconductor wafer from which it was fabricated. Ordinarily VCSELs are manufactured with many layers of semiconductor material deposited upon the substrate. The VCSEL includes highly reflective optical mirrors above and below the active layer, which enable laser output normal to the surface of the wafer.
It has been observed that optoelectronic devices, such as VCSELs, light emitting diodes, resonant cavity photodetectors (RCD) and other devices, when encapsulated in a material with an index of refraction other than air, such as plastic or epoxy, exhibit properties that differ compared to the same device in air. The properties that change include the threshold current and slope efficiencies in the case of a laser or resonance depth and bandwidth in the case of an RCD. The reason for the change is that the index of refraction for air, or a vacuum, is 1 while the index of refraction for plastic or glass, for example, is approximately 1.5. The transmission from the top surface of the device is therefore changed when it is embedded in a different index, changing the device characteristics.
Generally, an advantage of the VCSEL and other surface-normal devices is that it can be tested and characterized while still part of the wafer. Such automated testing is very efficient, enabling characterization and screening of properties such as the resistance, slope efficiency and threshold current over the operating temperature range. If the facet transmission changes after encapsulation, however, the slope efficiency and threshold current will change, making the prior testing inaccurate. This change in properties upon encapsulation therefore requires additional testing or careful test correlation and control of the laser properties affecting the changes in performance with changing transmission upon encapsulation.